During blood flow, fluid mechanical forces cause increased numbers of platelets at the wall compared with predictions from platelet counts of bulk, static blood. The flow dependence of the increased number was investigated in previous work; it indicates that the wall concentration is maximum at an intermediate value of the wall shear rate. The proposed work uses the generalized Fahraeus effect, which occurs in capillary tubes, to experimentally investigate the flow dependence of the wall concentration of platelets. Specific methods are described by which experimental blood samples can be accurately collected and evaluated on a Coulter counter. Variables to be explored in the experiments include wall shear rate, tube diameter, axial position and blood-suspension characteristics such as platelet-red-cell size ratio, plasma viscosity and red cell flexibility. Specific aims are to verify entry length and elaborate the fluid mechanical mechanisms that cause increased platelet concentrations. Experimental results of the tube platelet count will be interpreted via two models of platelet wall concentration. The first is a one-dimensional, integral form of mass-conservation for multi-phase flow with appropriate assumptions to make the model determinate. In the second model of mass conservation in a flowing system, the platelet concentration in the marginal layer is controlled by three competing fluxes, which are driven by the repulsive, shear-induced wall force and gradients of diffusivity and concentration. Diffusion in the marginal layer is treated as a combination of Brownian and shear-enhanced diffusion. The results of the proposed work will further the understanding of the general relationship among flow, thrombosis, and sub-lethal platelet damage by showing how flow changes the number of patelets that are exposed to the wall. The results will aid rational design of tests for blood-compatible biomaterials and artificial internal organs.